This invention relates to electric power system relaying, and, more particularly, to high-speed relays for controlling the isolation of short circuits which occur on power system transmission lines.
As power systems have grown in the power transmission industry, the time available for protective equipment to remove faults and still maintain power system stability has been significantly reduced. There are two ways of keeping pace with system stability requirements; either faster relays or faster circuit breakers are required. The instant invention is directed to reducing relay time.
Conventional transmission line relays have reached an apparent response time limit of about 4 milliseconds, with a state-of-the-art limit of about 2.5 msec. To achieve faster response time in these conventional systems, system security would have to be comprised. System security requires that spurious operation inputs be ignored, so that system circuit breakers are not opened incorrectly during system disturbances that normally occur, but which do not interfere with system performance.
One approach to reducing relay time employs traveling wave concepts. These methods have been described by Dommel et al., "High Speed Relaying Using Traveling Wave Transient Analysis", Paper A-78-214-9, IEEE PES 1978 Winter Meeting, New York, NY and by Esztergalyos et al., "Development, Design, Application and Field Experience of an Ultra High Speed Relaying System for EHV/UHV Transmission Lines", Paper presented at the Pennsylvania Electric Association Relay Committee Meeting, October, 1978, Harrisburg, Pa. Because of the possibility of spurious input signals, the first wavefront to be received at the relay employing traveling wave concepts cannot be used by itself to make an operating decision. Some form of filtering, integrating or averaging of the signal must be done. The result is that the schemes respond not only to the first wavefront, but also to subsequent reflections of the first wavefront. Furthermore, since traveling waves can propagate through an entire power system, traveling wave schemes do not determine fault location. Although traveling wave schemes can be made to have distance properties by introducing a threshold, determining its magnitude precisely can be done only by extensive computer simulations. For these and other reasons including mutual coupling between adjacent lines on the same right-of-way, traveling wave schemes are not secure and are subject to generating incorrect trip signals during normal power system disturbances, such as switching surges and loss of communications channels, and can also trip the wrong circuit breakers during genuine short circuits.
Other techniques have been employed to achieve high speed relaying on transmission lines. For example, a fault simulation technique described in British Pat. No. 1,517,551, issued to Hughes Aircraft Co. on July 12, 1978, can rapidly detect single line to ground faults by comparing measured waveforms with simulated waveforms. However, the technique does not work well for multi-phase faults. As an example of a digital scheme, Takagi et al., "Digital Differential Relaying System for Transmission Line Primary Protection Using Traveling Wave Theory--Its Theory and Field Experience", Paper A-79-096-9, IEEE PES 1979 Winter Meeting, New York, NY, describe a method which compares the currents at opposite ends of the transmission line. However, the method cannot provide first zone distance protection without a reliable communications channel to link the line terminals.